drivers/modem/ublox-sara: Fix build warning

Fix the following build warning:
include <fcntl.h> without CONFIG_POSIX_API
is deprecated. Please use CONFIG_POSIX_API
or #include <zephyr/posix/fcntl.h>

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>